1. Field of the Invention
This invention relates to a system and method for depositing oxide and other inorganic materials onto the inside surface of 3 dimensional shapes, such as bottles, at high rates appropriate for production environments.
2. Description of the Related Art
One of the greatest challenges in the plastics packaging business has been the reduction of gas transfer through polymeric materials to either stop gases from ingressing into the packaged product, or to stop gases from egressing from the packaged product. There have been many approaches attempted including new resin formulations and multi-layers of polymeric materials, but each has had problems finding widespread acceptance due to either the cost, non-recyclability or the performance.
Traditionally, polyester terepthalate (PET) is the polymer of choice when gas barrier is needed in plastic packaging. In the three-dimensional (or rigid packaging area), PET is used in almost all applications where shelf-life and clarity are required.
Rigid packaging, sometimes called three-dimensional packaging, includes bottles, cans, cups and typically excludes the so-called flexible packaging. Examples of flexible packaging include pouches, and bags.
Although widely used in rigid packaging, PET is limited in its ability to provide gas barrier to both gas coming into the product (gas ingress) and escaping (gas egress). In the case of beer, a highly oxygen sensitive beverage, even the oxygen that is adsorbed in the wall of the PET bottle can significantly alter the taste and shelf-life of the beer. For carbonated soft drinks (CSD), on the other hand, the barrier must stop carbon dioxide from escaping out of the beverage and there are little to no concerns about the ingress of gases.
There are three conventional approaches to providing barriers in PET bottles; multi-layers, mono-layers, and surface coatings. The first approach is to provide a multi-layer structure that sandwiches the PET structural layers around a core, a single layer or multiple layers, containing higher-priced barrier materials.
This first approach has been utilized by incorporating ethylene vinyl alcohol (EVOH) as the core. EVOH provides excellent oxygen barrier properties, however, EVOH is highly moisture sensitive and the barrier properties deteriorate with exposure to water. Other materials such as nylon have also been investigated but have had issues with recyclability and cost.
There are several new barrier materials including nylon-based nanocomposites and “passive-active” barrier systems. The passive-active barrier systems include dual-acting formulations of a passive barrier material and an active oxygen scavenger that blocks oxygen by adsorbing it chemically. Both of these materials have been successfully integrated into multi-layer structures, however, none have been demonstrated to be recyclable, and due to the high cost of the barrier materials, the overall cost of the package is outside of acceptable limits.
The “ideal” route to providing barriers in PET bottles (and the second conventional approach) is to use a monolayer that either incorporates a barrier material (mixed with PET to reduce cost) or is formed of a single polymer. There are, however, few practical monolayer materials under development. Further, there is still the significant issue of recycle of the monolayer, since the current recycle streams for polymers are set-up to handle only polyolefins (which offer no barrier to oxygen) and PET. Any composite material (incorporating PET) or other polymer would potentially contaminate the current recycle streams.
The third conventional approach to providing barriers in PET bottles is surface coating technologies where a thin layer is applied to either the exterior or interior surface of the PET bottle. If a polymer is used, the same issues discussed above are present for recycle. Thus, the current focus of this area is the deposition of thin film coatings of silicon oxide or a carbon based material.
Thin films are defined as coatings that are measured on the angstrom (Å) level. Typically, the thin film coatings discussed as barrier materials range in thickness from 100-5000 Å in total thickness.
There are two approaches currently being developed for the deposition of the thin film coatings, physical vapor deposition and chemical vapor deposition. For examples of physical vapor deposition techniques, see U.S. Pat. Nos. 6,279,505, 6,276,296, 6,251,233, 6,223,683, 6,520,318.
Physical vapor deposition techniques have not been successful due to the application of the thin film onto the exterior surface of the bottle and the inherent damage that the thin film incurs during subsequent processing such as during filling, labeling, capping and transportation. To prevent this damage to the thin film, a second additional coating that protects the thin film from damage was sprayed onto the thin film. This additional coating adds processing time and increases the cost of the barrier technology and final package.
The second approach currently being developed for the deposition of the thin film coatings is by chemical vapor deposition utilizing plasma enhanced chemical vapor deposition (PECVD), where the coating is derived from gases that are decomposed within the bottle by a plasma. The plasma, an electrically ionized gas, is created by coupling power into the gas mixture (held at a pressure significantly lower than atmospheric pressure) through an electric field. The PECVD approach can be broken down further into carbon and silicon based chemistries. For examples of carbon based and silicon based coatings see U.S. Pat. No. 6,294,226 and U.S. Pat. Nos. 6,565,791, 6,117,243, 5,972,436, 5,900,285, 6,390,020, 6,055,929, 5,993,598, 5,900,284, 6,180,191 and 6,112,695.
In the application of the surface coating technologies, one of the key performance factors is the speed at which the bottles are coated. The coating speed should be near the rate at which bottles are produced from blow molding, which currently averages around 20,000 bottles per hour (bph), but can be as high as 60,000 bph.
If the coating speed cannot achieve at least 20,000 bph and be extendable to at least 40,000 bph, there will be significant limitations in terms of the utilization efficiency of the apparatus. With current trends, a desirable technology should be extendable to 40,000 bph. As will be discussed below, one significant limitation of existing surface coating applications based on thin film materials (carbon or silicon based), is that no commercial equipment can be extended beyond 10,000 bph.
To date, each bottle that is coated on the interior with a thin film coating has been treated as a separate entity. In this approach, the interior of each bottle and the surrounding environment are evacuated (with a vacuum pump) to process pressures, then gases are injected into the bottle and a plasma is ignited. The plasma, in contact with the interior of the plastic bottle, results in the thin film coating.
Once the coating process is completed, the bottle is evacuated a second time, and then vented to atmospheric pressure. Since each bottle is treated independent of other bottles, pumping, gas and power systems must be duplicated for each individual bottle reactor. This results in duplication of costs, but more importantly, results in significant increases in process cycle time (defined as the time from when a bottle enters the machine until it exits with a thin film coating on the inside). In addition, the only means to increase the number of bottles processed through the machine (assuming that the coating time is constant), is to increase the number of coating cells and associated hardware. This results in significant increases in the size and cost of the equipment.